1. Field of the Invention
The present invention relates to a drum brake assembly carried on a vehicle, and more particularly to a drum brake in which the structure of a wheel cylinder for expanding the brake shoes is simplified and made compact, whereby not only the cost to manufacture the drum brake and the weight of the same are reduced but also a stable braking force can be obtained.
2. Description of the Related Art
A conventional drum brake generally comprises an input portion (wheel cylinder), brake shoes which are expanded with a force received from the input portion to generate a braking force, and an anchor portion which supports the brake shoes and receives an anchor reaction force.
Various types of drum brakes have been used for controlling the traveling of vehicles. These drum brakes are classified into a leading trailing type, a two leading shoe type, a duo-servo type and so on.
Of these types of drum brakes, the leading trailing type of a drum brake is constructed such that a couple of brake shoes are oppositely disposed, a wheel cylinder for expanding the brake shoes is disposed between first ends of the shoes, which are opposite to each other, an anchor-portion is disposed between second ends of the brake shoes, and a leading shoe and a trailing shoe are assembled into the drum brake structure. The leading trailing drum brake is advantageous in that the braking force is stably produced and assembling of the parking brake into the drum brake structure is easy.
The two leading shoe type of drum brake includes a couple of leading shoes assembled thereinto. This type of drum brake is further classified into a single drive type (TP1W) and a dual drive type (TP2W). The TP1W drum brake exhibits a high gain of braking force when the brake drum is rotated in the forward direction. In a backward direction of the drum rotation, the drum brake operates as the drum brake of the two trailing shoe type, and exhibits a smaller braking force gain than in the forward direction. The TP2W drum brake has a high braking force gain in both the forward and backward directions.
The duo-servo drum brake includes two brake shoes, primary and secondary shoes, which are linked with each other. A braking force produced by the primary shoe is input to the secondary shoe. The duo-servo drum brake produces a braking force at high gain in both the forward and backward directions.
The two leading shoe type of drum brake requires two wheel cylinders. Because of this, this type of drum brake is high in cost and needs a complexity of the parking brake mechanism.
The duo-servo drum brake produces an extremely high braking effect when comparing with the leading trailing type of drum brake and the two leading shoe type of drum brake. Further, the size reduction of this type of drum brake is easy, and the assembling of the parking brake into the brake structure is easy. However, the duo-servo drum brake is disadvantageous in that it is sensitive to variations of the frictional coefficients of the lining of the brake shoes and of a contact state variation of the rotary brake drum, and hence the braking effect produced by the drum brake greatly varies, providing instable braking characteristics. This problem is serious.
The leading trailing type of drum brake is superior to the remaining ones in that the braking force produced is stable, the assembling of the parking brake is easy, but inferior to the latter in that the braking effect obtained is low. A conventional measure taken for overcoming the insufficient braking effect problem is to increase the performance of the brake booster for the master cylinder and to increase the diameter of the brake drum. The measure makes it difficult to reduce the size of the drum brake.
Thus, those types of conventional drum brakes have advantages and disadvantages. A suitable type of drum brake is properly selected from among those types of drum brakes in accordance with drive performances, use purposes, and production scale and other factors of the vehicles into which the drum brake is incorporated.
In recent, remarkably advanced vehicles, how to improve the braking effect of the drum brake is of significance.
Various approaches to solve the instable braking effect of the duo-servo drum brake have been made and attracts engineer's attention.
An attractive approach of those ones is that the anchor reaction force coming from the brake shoe is used to control the operation of the wheel cylinder for expanding the brake shoes.
Also in the duo-servo drum brake, a wheel cylinder is disposed close to the anchor pin. A pair of hydraulic pistons are extended from the wheel cylinder to the primary and secondary shoes. The primary and secondary shoes are expanded by the hydraulic pistons to be pressed against the brake drum. A highly sophisticated control of hydraulic pressure is essential to stabilize braking force. This hinders simplification of the whole mechanism of the duo-servo drum brake.
Further, in the duo-servo drum brake of which the brake shoes are worn, extremely delicate work is required for the clearance adjustment for the primary and secondary shoes, and sometimes it is necessary to manually adjust the clearances. In this respect, the duo-servo drum brake so as to be little affected by such a clearance adjustment is desirable.
If a brake design is made to control the operation of the wheel cylinder by the anchor reaction force and it needs structural complexity of the wheel cylinder, the size of the wheel cylinder is increased. The increased size of the wheel cylinder leads to increase of size and weight of the drum brake, and increase of cost of the wheel cylinder and the brake per se. The duo-servo drum brake may be constructed such that the anchor reaction force acts on the cylinder body of the wheel cylinder to effect the control of the wheel cylinder operation by the anchor reaction force. In this case, highly rigid and expensive materials must be used for making the cylinder body. This also brings about the cost and size increase of the wheel cylinder.